1. Field of the Invention
The present invention provides a recombinant herpesvirus of turkeys (rHVT) comprising cDNA of the F protein (F gene) of Newcastle disease virus (NDV) under the control of a modified chicken beta-actin promoter.
2. Description of the Related Art
Newcastle disease is one of the most fearful contagious diseases in the poultry industry. Newcastle disease presents itself in many forms ranging from high mortality to an asymptomatic form. Strains are classified as (1) velogenic (high-virulence), (2) mesogenic (moderate-virulence), (3) lentogenic (low-virulence), and (4) asymptomatic (Alexander, D. J. In Diseases of Poultry 1997). Chickens infected with velognenic forms of NDV become gloomy and lethargic in a few days and the mortality rate is from dozens to more than fifty percent. Surviving birds often develop neurological symptoms such as wryneck or gyrospasm. One of the reasons the disease is so fearsome is that chickens are susceptible to NDV regardless of age and those infected with velogenic NDV show fulminant symptoms at all ages. Since NDV is highly transmissible, every chicken must be disposed of at the outbreak of the disease. The hennery should thoroughly be disinfected to prevent further infection. Strains in the mesogenic pathotype (moderate-virulence) are characterized by death in young chickens. Strains in the lentogenic pathotype (low-virulence) are characterized by mild respiratory infections and many of these strains are used to prepare vaccines for use in young chickens. Asymptomatic enteric strains are usually isolated from the gut of chickens showing no disease (Alexander, 1997). Newcastle disease affects both chickens and turkeys, however the clinical signs in turkeys are less severe than in chickens (Alexander, 1997).
At present, live and inactivated vaccines are available for the prevention of Newcastle disease. These vaccines are effective but not free from defect. The inactivated vaccine must be inoculated into breeder hens in lay, repetitively. The live vaccine is mainly for young chickens. However, long lasting immunity is not guaranteed for young chickens, which have high maternal antibody levels. Repeated administration of live vaccines is sometimes detrimental to the healthy growth of young chickens due to vaccine reactions causing mild respiratory disease. Thus, a new type of vaccine, which is efficacious, free from adverse effects, and does not require repeated administration, is desirable for the poultry industry.
To meet the industry's wishes, Sakaguchi et al. (J. Virol. 74:3217-3226, 2000; Vaccine 16:472-479, 1998) developed a recombinant Marek's disease virus serotype 1, which had the NDV F gene in the US10 region of the virus genome. The obtained recombinant virus induced lasting protective immunity against Newcastle disease in SPF chickens as well as in maternal antibody positive commercial birds.
Marek's disease is another serious problem for the poultry industry. Against this disease, herpesvirus of turkeys (HVT) (Witter R. L. et al. Am. J. Vet. Res. 1970, 31, 525-538) has been most widely used as a safe vaccine.
Until now, there have been several reports about the HVT-based Newcastle disease-Marek's disease bivalent vaccine. For instance, Morgan et al. (Avian Dis. 37:1032-1040, 1993; Vaccine 11:349-358, 1993; Avian Dis. 36:858-870, 1992) constructed recombinant HVTs having the NDV F gene and examined the efficacy of these recombinants as a Newcastle disease vaccine. Macmillan et al. (Vaccine 14:469-477, 1996) constructed recombinant HVTs expressing HN and F proteins of NDV and tested these recombinants for efficacy. In both cases, vaccinated SPF chickens were protected against NDV, but not satisfactorily commercial chickens having high maternal antibody levels
Saitoh et al. inserted cDNA encoding F and HN proteins of NDV into the HVT genome (WO 99/18215). The inserted genes were under the control of the CMV or RSV promoter. The foreign gene insertion site was a newly identified intergenic region between UL44 and 45 or between UL45 and 46. These recombinants conferred good protection against NDV challenge in SPF chickens as well as in chickens with NDV maternal antibodies. However, these recombinants expressed two inserted genes of HN and F. Since HN protein induces heamagulutination inhibition (HI) antibodies to immunized chickens, it is difficult to distinguish vaccine immunized chickens and NDV infected chickens. Therefore, a recombinant virus expressing F protein gene, which induces an adequate protective immunity against Newcastle disease, is a more desirable vaccine. This objective was not easily attainable because rHVT having only F gene of NDV didn't induce desirable immunity in chickens as indicated by Morgan et al.